The invention relates to detectors of ionizing radiations, especially X-rays, and more particularly gas detectors, that is, those for which the material absorbing the radiations for generating electrons is a gas (for example comprising argon or xenon).
This type of detector is for example used for analyzing samples of material (metal alloys, proteins, crystalline structures, biological macromolecules, etc.) in order to determine the structure thereof. The samples are placed in front of the detector and laterally lighted (as a rule) by an X-ray source; they diffract the radiations and send them back to the detector and the function of the latter is to determine the angle of incidence according to which it receives the X-rays, that is, the diffraction angle due to the sample. The measured diffraction angles supply information on the structure of the sample material.
The known two-dimensional gas detectors have a structure which is generally shown in FIG. 1. They correspond for example to what is described in FIG. 1 of U.S. Pat. No. 4,595,834.
The detector comprises an air-tight chamber 10 containing the absorbing gas and, on the rear face, an airtight input window 12, transparent to the X-rays. This window carries a transparent electrode 14 set to a voltage V1. Between the window 12 and the bottom of the chamber 10 a space 16 extends, called absorption and drift space, filled with gas (argon or xenon with polyatomic additives).
At the bottom of the chamber, opposite to window 12, an electron detector 18 is placed, called "localization detector" because of its function which is to detect the presence and the position of an electron package originating from the ionization of the gas in the chamber. This detector 18 comprises an electron-transparent input electrode 19, set to a voltage V2 higher than V1 (for example 0 volt if V1 is at -4,000 volts and if the distance between the electrodes 14 and 19 is about 10 cm).
A sample of material 20, placed outside the chamber, in front of window 12 and at a determined distance of the latter, is laterally lighted by an X-ray source 22.
Through diffraction, a photonic radiation 24 is reemitted from the sample towards the absorbing gas with an angle of incidence that is desired to be known.
After entering the gas, a photon will be absorbed at a point of the chamber and at this point it will emit an electron or an electron package. The electric field in the absorption and drift space is generated by the voltage difference V2-V1 so that the electrons may derive, along the field lines, towards detector 18 and their arrival position is detected. The field lines are straight lines perpendicular to electrodes 14 and 19.
As it will be seen in FIG. 1, according as the incident photon is absorbed at a point A or at a point B of its path, the electron detector 18 will detect an arrival position a or b of an electron package.
This means that it is not possible to univocally determine the incidence angle of the radiation 24 from the arrival position.
There is a so-called parallax error because the electric field which causes the electrons to derive is not orientated in the same direction as the incident ray 24.